This invention relates generally to electro-acoustic transducers and more particularly to cylindrical transducers having insulation structures.
As is known in the art, a transducer is a device that converts energy from one form to another. In underwater acoustic systems, transducers generally are used to provide an electrical output signal in response to an acoustic input which propagates through a body of water or an acoustic output into the body of water in response to an input electrical signal.
An underwater acoustic transducer designed primarily for producing an electrical output in response to an acoustic input is called a hydrophone. Hydrophones are typically designed to operate over broad frequency ranges and are also generally small in size relative to the wavelength of the highest intended operating frequency.
A transducer intended primarily for the generation of an acoustic output signal in response to an electrical input is generally referred to as a projector. Projector dimensions are typically of the same order of magnitude as the operating wavelength of the projector. Moreover, projectors are generally narrowband devices, particularly compared to hydrophones. Both hydrophone and projector transducers are widely employed in sonar systems used for submarine and surface-ship applications.
Projectors generally include a mechanically driven member such as a piston, tube, or cylinder and a driver. The driver is responsive to electrical energy and converts such energy into mechanical energy to drive the mechanically driven member. The driven member converts the mechanical energy into acoustic waves which propagate in the body of water. Most acoustic transducers have driver elements which use materials having either magnetostrictive or piezoelectric properties. Magnetostrictive materials change dimension in the presence of an applied magnetic field, whereas piezoelectric materials undergo mechanical deformation in the presence of an electrical field. Because ceramic materials used in piezoelectric ceramic drivers are generally incapable of supporting tensile stresses, which often leads to fracturing of the ceramic, it is generally required that the ceramic driver be placed in a condition of precompression or prestress. Precompression protects the ceramic element from tensile forces which are generally detrimental to ceramic piezoelectrics.
Because acoustic transducers are used in a wide variety of applications, their size, shape and mode of operation can be quite different.
A configuration for acoustic transducers used when light weight and small size is needed is the split-ring cylindrical transducer. The split-ring transducer generally includes a continuous hollow tube having a longitudinal gap extending the length of the tube. The transducer also includes a cylindrical ceramic driver having a longitudinal gap at an angular displacement, such that when the driver is disposed within the tube, the respective gaps are generally aligned. In one configuration, a cylindrical ceramic driver has electrodes on the inner and outer surfaces and is polarized in a manner such that when an alternating current is applied across the electrodes, the driver causes the hollow tube to expand and contract in the radial direction. Accordingly, the ceramic driver and the hoop-mode projector are said to operate in the radial mode. The "C" shaped projector vibrates similarly to a tuning fork with the motion of the centers of vibration on either side of the diametral plane of the split having a large displacement normal to the plane as compared to the point diametrically opposite the split, which has a relatively small displacement.
Prestress is applied to the cylindrical ceramic driver by using a split hollow tube having a diameter somewhat smaller than the diameter of the ceramic cylinder driver. The opposing arms or tines of the tube are spread apart sufficiently for inserting the cylindrical ceramic element within the tube. Releasing the spreading forces on the opposing arms allows the tube to wrap itself around the ceramic driver and places the driver in compression. The resonant frequency of the split-ring projector is a function of the diameter as well as the thickness and elasticity modulus of the tube and ceramic driver materials.
There are several different types of piezoelectric ceramic drivers commonly used for split cylinder transducers.
The simplest and most common configuration uses the aforementioned cylindrical ceramic driver having electrodes on inner and outer surfaces of the cylinder. In applications where the cylindrical ceramic driver is unable to provide sufficient drive for a higher acoustic power split cylinder transducer, a barrel stave cylindrical driver may be used. Barrel stave driven transducers are known to provide as much as a 6 dB improvement in output acoustic power. The barrel stave driver includes a plurality of segmented sections of ceramic having electrodes disposed between individual ceramic segments. The polarity of the ceramic segments are alternated at every other electrode such that when driven with an alternating current, the driver causes the split cylinder tube to expand and contract circumferentially. The barrel stave cylindrical driver is said to operate in the circumferential mode.
A cylindrical ceramic driver which operates similarly to the barrel stave driver is the tangentially-poled ceramic driver. The tangentially-poled driver includes a cylindrical ceramic tube having a plurality of electrically conductive stripes disposed at predetermined and generally equally spaced locations around the circumference of the driver, with each conductor stripe forming a circuitous closed path around the wall of the driver. The electrically conductive stripes are generally painted on with a conductive paint, such as silver paint, and they are typically cured by a furnace firing process. In this configuration, the tangentially-poled ceramic driver appears and operates similarly to the barrel-stave ceramic driver.
One problem with both the barrel stave and tangentially-poled cylindrical drivers is that when placed in a metal shell, electrical continuity is provided between the ceramic driver and the shell, resulting in a short-circuited and inoperative transducer.
Further, the relatively high voltages generally needed for driving the ceramic elements require that any insulative layer provided between the shell and ceramic elements, must have a sufficiently high dielectric constant for preventing voltage breakdown and arcing through the layers and concurrently, that the insulative layer be sufficiently thin for providing as much of the energy generated by the ceramic to the shell rather than the energy being dissipated within the insulative layer.
One solution to this problem would be to use a shell fabricated from a non-conductive material. However, non-conductive shells having similar dimensions for use in a split-ring cylindrical transducer typically are fabricated from materials having a value of elasticity modulus that is generally inappropriate for use in certain applications, such as in a sonobuoy sonar systems. Further, the manufacturing costs of non-conductive shells is generally higher than for metal shells.